Ferromagnetic interactions in hosted bipartite materials—generalized-double-exchange and generalized-superexchange interactions

• Published in: Journal of physics. Condensed matter Vol. 23; no. 8; pp. 086004 - 8
• Main Authors: Andriotis, Antonis N, Lisenkov, Sergey, Menon, Madhu
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 02.03.2011; Institute of Physics
• Subjects: Codopants; Condensed matter; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Exchange and superexchange interactions; Magnetic properties and materials; Magnetic semiconductors; Magnetically ordered materials: other intrinsic properties; Magnetism; Molecular structure; Nitrides; Percolation; Physics; Studies of specific magnetic materials; Thresholds; Double exchange; Fullerene polymer; Long-range order; Magnetic moments; Molecular interaction; Spin density; Gallium nitride; Semimagnetic semiconductors; Impurities; Polarized spin; Superexchange interactions; Density functional method; Zinc oxide; Generalized gradient approximation; Exchange interactions
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/23/8/086004

Defect-induced magnetism in dilute magnetic semiconductors challenges our understanding of magnetism in solids. Theories based on conventional superexchange or double-exchange interactions cannot explain long range magnetic order at concentrations below the percolation threshold in these materials. On the other hand, the codoping-induced magnetism, which can explain magnetic interactions below the percolation threshold, has eluded explanation. In this work we propose that defect-induced magnetism in codoped non-magnetic materials can be viewed within a molecular generalization of the atomic double-exchange and superexchange interactions applied to an arbitrary bipartite lattice hosted by (or embedded in) defect-free non-magnetic materials. In this view, the crucial factor for the development of magnetism appears to be the defect complementarity of the codopants. We demonstrate this by taking ZnO and GaN (the most widely studied doped oxide and nitride magnetic semiconductors, respectively) as host materials and perform theoretical calculations using ab initio methods after codoping them with transition metal impurities for a variety of configurations. Our results indicate that the magnetic coupling among the induced and/or doped magnetic moments takes the form of an interaction among spin-polarized molecular units which is facilitated by the formation of the hosted bipartite codopant structures. The universality of the proposed mechanism is further supported by earlier results referring to the rhombohedral C(60)-based polymers.